The present invention relates to the field of methods and installations for the cryogenic treatment of products, particularly food products, the treatment in question being in particular crust-freezing (deep-freezing some or all of the surface of the product), cooling or deep-freezing treatments.
Food products are conventionally frozen in freezing tunnels where the cooling is obtained by mechanical means.
These food products to be frozen are often tacky and adhere to the conveyor belts of the freezing tunnel on which they are carried, thus posing a problem of maintenance and hygiene.
Furthermore, these products may not be very compact and may readily deform, thereby losing their intended shape when they are handled. For example, this is the case with vegetable croquettes which are extremely difficult to handle.
In document EP-A-505 222, the Applicant proposed a novel concept for a method of freezing food products, according to which the product is brought in contact with a refrigerating surface which results from the use of a vibrating support and a liquefied gas, the refrigerating surface consisting of a liquefied gas film arranged on the support.
According to this prior art, the products do not adhere at all to the support even if they are very tacky, despite the thickness of the film possibly being very small, and it seems very likely that the product thus treated floats on the surface of the liquefied gas film by a calefaction effect, and regularly turns over in this film so as to avoid any risk of adhesion to the support.
Typically, this system operates in the following way: a large quantity of liquid nitrogen is injected into the container, which is for example configured with a slightly upward slope. The liquid overspill leaves the apparatus with the products. The nitrogen is then separated from the products by a grille located at the exit of the device. The nitrogen recovered in this way is recycled: it is collected in a reservoir then pumped by a piston pump and returns to the treatment container.
The level of nitrogen is kept substantially constant in the reservoir owing to a valve driven by a probe, which measures the liquid nitrogen level therein.
The nitrogen thus flows in a semi-closed circuit, and leaves the circuit only by evaporation in contact with the products; this loss of nitrogen is compensated for continuously by the supply of the reservoir. The products pass through the container only once.
It should be emphasized that this prior art system has a number of advantages, including:                the level of liquid nitrogen is stable;        the treatment of the products is uniform;        the intensity of the treatment can be adjusted by modifying the slope of the container;        the treatment time can be adjusted by modifying the amplitude of the vibrations;        the principle is simple, easy to use and easy to regulate;        the substantial injection of nitrogen into the container (injection rate=pumping rate) makes it possible to obtain very efficient treatment of the products.        
Nevertheless, the Applicant has now realized that this system should be improved, particularly regarding the following aspects:                certain drawbacks have been found, associated with the presence of the pump which represents the critical element of the system: this pump consumes a non-negligible quantity of compressed air and, when the throughput of the products to be treated is very large, limits the overall cooling capacity of the system by its pumping capacity,        furthermore, the system poses problems for small and powdered products: this is because the size of the product may become less than the size of the holes in the grille, so that it flows in a closed circuit with the nitrogen, which is clearly unsatisfactory in terms of sanitation.        
It can therefore be seen that the drawbacks listed above are essentially connected with the presence of the pump.